The present invention relates generally to drop-on-demand liquid emission devices such as, for example, ink jet printers, and more particularly such devices which employ an electrostatic actuator for driving liquid from the device.
Drop-on-demand (DOD) liquid emission devices with electrostatic actuators are known for ink printing systems. U.S. Pat. Nos. 5,644,341 and 5,668,579, which issued to Fujii et al. on Jul. 1, 1997 and Sep. 16, 1997, respectively, disclose such devices having electrostatic actuators composed of a diaphragm and opposed electrode. The diaphragm is distorted by application of a first voltage to the electrode. Relaxation of the diaphragm expels an ink droplet from the device. Other devices that operate on the principle of electrostatic attraction are disclosed in U.S. Pat. Nos. 5,739,831, 6,127,198, and 6,318,841; and in U.S. Pub. No. 2001/0023523.
According to the prior art, an electrostatic attraction force is applied in a single direction, as the electrodes can only attract; repulsion being impossible. Thus, the devices must rely on the elastic memory of the diaphragm to return to an at-rest position. In order to produce sufficient force, large electrodes are required, and the gap between electrodes needs to be small. These two criteria are difficult to achieve while still providing for sufficient displacement to expel a reasonably sized droplet. Another drawback of large electrodes is the poor spatial resolution between nozzles.
Devices that rely on the elastic memory of the diaphragm to expel liquid drops exhibit a reduction on the force over the time that liquid is being expelled. That is, the speed at which the diaphragm moves as it approached its at-rest position decreases. The result is a tendency toward the production of undesirable satellite droplets accompanying the main drop.
According to a feature of the present invention, a drop-on-demand liquid emission device, such as for example an inkjet printer, includes an electrostatic drop ejection mechanism that employs an electric field for driving liquid from the device. Structurally coupled, separately addressable dual electrodes greatly enhance the fundamental efficiency of the electrostatic drop ejection mechanism. The increased efficiency of the electrostatic drop ejection mechanism enables a reduction of electrode size (area) and reduces the required electrode voltage.
The liquid emission device includes a liquid chamber having a nozzle orifice. Separately addressable dual electrodes are positioned on opposite sides of a single ground electrode such that the three electrodes are generally axially aligned with the nozzle orifice. The ground electrode is structurally stiff, and the two addressable electrodes are structurally connected via a rigid, electrically insulating coupler. To eject a drop, an electrostatic charge is applied to the addressable electrode nearest to the nozzle orifice, which pulls that electrode toward the ground electrode and away from the orifice. This electrode forms a wall portion of the liquid chamber behind the nozzle orifice, so that movement of this electrode away from the nozzle expands the chamber, drawing liquid into the expanding chamber. The other addressable electrode moves in conjunction, storing elastic potential energy in the system. Subsequently the addressable electrode nearest to the nozzle is de-energized and the other addressable electrode is energized, causing the other electrode to be pulled toward the ground electrode in conjunction with the release of the stored elastic potential energy. This action pressurizes the liquid in the chamber behind the nozzle orifice, causing a drop to be ejected from the nozzle orifice.
There are several advantages associated with the present invention. The efficiency of the electrostatic drop ejection mechanism will be increased. The force applied during the final stages of drop ejection and separation will be positive and controllable such that the risk of satellite formation is substantially reduced. Since there is no electric field across the ink, conductive inks and other liquids can be used. Also, the electric field can be across air or other dielectric fluid, enhancing the electrostatic performance of the system.